The present invention relates to the manufacture of polyacrylic acids, or their salts, having a low molecular weight. More precisely, it relates to the preparation of concentrated aqueous solutions of low molecular weight polyacrylic acids or their salts.
The preparation of polyacrylic acids by free radical polymerization of the corresponding acid has been known for a very long time and is carried out in accordance with various processes (see Kirk-Othmer, "Encyclopedia of Chemical Technology", 2nd Edition, Interscience Public Publishers 1963, Volume I, page 306).
A conventional method consists in polymerizing acrylic acid in aqueous solution or in an organic solvent medium, such as, for example, benzene. In general, the polymerisation of acrylic acid is carried out in the aqueous phase in the presence of initiators, such as hydrogen peroxide, sodium persulphate or potassium persulphate or organic peroxides, or the polymerization is initiated with a redox system composed, for example, of potassium persulphate and sodium thiosulphate.
It is convenient to carry out the polymerization, in aqueous solution, using solutions having a monomer concentration of less than 25%, because, in the case of more concentrated solutions, the polymerization is difficult to control as a result of the great heat of polymerization evolved.
A traditional method of controlling molecular weights is to introduce an agent for limiting the length of the polymer chains, and U.S. Pat. No. 2,789,099, which describes the use, for fulfilling this purpose, of a system composed of sodium hypophosphite and copper acetate, may be mentioned.
It is also possible to employ alcohols as chain-transfer agents, but the disadvantage of a process of this type is that partial esterification of the polyacrylic acid takes place, and this does not provide the desired objective.
A new process, which comprises the present invention, has now been provided which makes it possible to control the molecular weights obtained and which thus leads to the preparation of polyacrylic acids having low molecular weights and a narrow distribution of these weights.
It is accordingly, an objective of the present invention to provide a novel process for producing polyacrylic acid, or its salts, of controlled low molecular weight and a narrow molecular weight distribution.
It is also an object of the present invention to prepare polyacrylic acid containing aqueous solutions, in which the polyacrylic acid, or its salts, is of low molecular weight and narrow molecular weight distribution.
Other objects will be apparent to those skilled in the art from the present description.